1. Field of the Invention
The present invention relates to a zoom lens barrel.
2. Description of the Related Art
In a known zoom lens barrel, a rotatable cam ring is provided on the inner peripheral surface thereof with a female helicoid (internal thread) which engages with a male helicoid (external thread) of a lens support cylinder which supports one of the front and rear lens groups and includes a bottomed cam groove in which a cam follower (pin) provided on a lens frame of the other lens group is fitted. If a helicoid is provided on the front lens group, resistance to external shock is increased and deviation of the lens from the optical axis can be prevented.
In a known zoom lens barrel, the bottomed cam groove penetrates in an area of the female helicoid in the axial direction of the cam ring. Therefore, the depth of the cam groove has to be deeper than the depth of the thread-groove of the female helicoid, and the thickness of the cam ring has to be determined taking into account the diameter of the female helicoid and the depth (diameter) of the cam groove, and consequently, hindering possible miniaturization of the lens barrel. Moreover, recent lens shutter type cameras have reached their limit, necessitating even smaller lens barrels.
It is possible to miniaturize a classic type lens barrel in which the cam ring has no female helicoid thereon and instead is provided with cam grooves for two or more lens frames. However, the classic type lens barrel has the drawback of insufficient resistance to external shock and the lens has the tendency to deviate or incline with respect to the optical axis.